MRI is moving inexorably toward higher field strength in search of improved signal-to-noise ratio and spectral resolution. At high fields (>4 Tesla) the precession frequency is higher and therefore the wavelength shorter. The tissue/field interactions become quite pronounced at high field and make obtaining high quality images very difficult. Beck has shown [Beck et al, MRM 51 Pg 1103-1107 2004] that at 11 T image intensity distributions are subject to significant distortions attributable to tissue/field interactions. Several methods such as optimizing the current distribution on the rungs of a multi-element volume coil [Ledden, Proc ISMRM 11 pg 2390 2003], transmitting field of time varying spatial characteristics using two separate pulses [Ledden & Cheng, Proc ISMRM 12 pg 38 2004] and the use of parallel imaging at high field strength [Weisinger et al, Proc ISMRM 12 Pg 323 2004] have been suggested to reduce the distortions.
A wide variety of parallel imaging techniques are available, such as the well known simultaneous acquisition of spatial harmonics (SMASH) [Sodikson & Manning, Magn Reson Med 38 pg 591-603, 1997] and sensitivity encoding (SENSE) [Pruessman et al, Magn Reson Med 42 pg 952-962, 1999]. Rapid parallel imaging is enabled by phased array coils as they allow scan time reduction compared to single coil acquisitions. The known techniques have typically used receive-only phased arrays but transceiver phased arrays are becoming more common.
Array technology was introduced by Roemer, as described in U.S. Pat. No. 4,825,162 assigned to General Electric Company. Roemer describes how images from an array of coils can be combined on a point-by-point basis to produce a single composite image. Roemer also describes how to minimise interactions between adjacent coils.
A significant amount of research directed to improving the usefulness on MRI has focussed on improving the signal to noise ratio (SNR). Larson described a focussing process in U.S. Pat. No. 5,252,922 that used a phased array of antennas to focus in a specific region of a body being imaged. Larson claims to have achieved focussing in volumes as small as 500-3000 cm3.
The SNR can be improved by applying higher fields but this can result in high Specific Absorption Rate (SAR), which may exceed regulatory guidelines. Peterson et al [Invest Rad 38 (7) pg 428-435 2003] has described a transceiver phased array for imaging the spinal cord at 3 Tesla. The Peterson system produced high resolution images. In a related paper [Invest Rad 38 (7) pg 443-451 2003], Peterson describes another transceiver phased array coil for imaging the pelvic region. Once again the phased array coils produced high resolution images.
Various array designs have been conceived for imaging various regions of the body. Persons skilled in the field will be familiar with common circular and rectangular array elements. Planar strip arrays are also known, such as described by Lee in United States patent application number 2003/0214299. Lee includes a useful background to MRI which is incorporated herein by reference. Lee also usefully notes that the current amplitude and phase of each transmit array element can be individually regulated such that the homogeneity of the RF excitation field is optimised in the presence of the patient.
Finally, reference may be had to International patent application number WO 2004/021025 filed by the present applicant. This patent application describes a coil array in which each coil element has its maximum sensitivity close to the centre of the object under study. The described coil array is useful for deep imaging of a body, for example cardiac imaging.